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PYROMETRY
415
imate value can be obtained by the use of an auxilliary thermometer, the bulb of which is placed about three-fourths of the way down the exposed mercury thread.
Tables 4, 5 and 5A show the magnitude of the emergent stem correction in some typical cases.
TABLE 5.—STEM CORRECTION DATA FOR DISTILLATION FLASK USED IN DISTILLATION OF CREOSOTE OiL1
Dimensions of flask: Diameter bulb 8 cm., diameter neck 2 cm., height neck 15 cm., distance bottom of outlet tube to top 8 cm. Thermometer range 0 to 400°C. in 1° intervals, length 40 cm.
Temperature of bulb, degrees Centigrade	Degrees of mercury column emergent	Mean temperature of mercury column	Stem correction
208	200	59	4.7
258	250	105	6.0
308	300	87	10.5
358	350	79	15.5
TABLE 5A.—STEM CORRECTION DATA FOR PENSKY-MARTIN FLASH-POINT APPARATUS
Thermometer used up to 150°C., range 40 to 160°C. in 1° intervals, scale length
9.5 cm.    Thermometer used from 200 to 300°, range 200 to 360° in 1° intervals,
scale length 12 cm.
Temperature of bulb, degrees Centigrade	Degrees of mercury column emergent, degrees Centigrade	Mean temperature of emergent mercury column, degrees Centigrade	Stem correction
50	30	35	0.1
100	80	45	0.7
150	130	55	2.0
200	10	75	0.2
250	60	85	1.6
300	110	100	3.5
Partial Immersion Thermometers.—As a means of avoiding the necessity for applying the correction for the emergent stem, it has become customary to use partial immersion thermometers which are graduated to read correctly when immersed to a definite depth, say three inches, in a bath. Such thermometers should be, and usually are, marked to show the proper depth of immersion, and many of them have a line engraved around the stem to show this depth of immersion. While the accuracy attainable with a partial immersion thermometer is not so high as can be attained with a total immersion thermometer under favorable conditions, it is true that these favorable conditions are not often realized, and therefore in the great majority of cases, the partial immersion thermometer is to be preferred. It is probable that the partial immersion thermometer will be extensively used in this country.
* See paper by Wilhelm, Bureau of Standards Tech. Paper No, 49.